While many different techniques for imaging are known, for candid photography using hand-held cameras the overwhelming choice is to employ a film that records images in one or more silver halide emulsion layer units coated on a film base. The film is most frequently purchased in a single or double roll cartridge that contains a film strip of sufficient length to provide 12, 24 or 36 exposure frames. More recently interest has revived in single use cameras containing preloaded film, first introduced before the turn of the century by George Eastman. By feeding the film from a tightly spooled roll for exposure and then again spooling the exposed film, the film and camera together can form a compact imaging unit.
Silver halide emulsions contain radiation sensitive microcrystals (grains) dispersed in a vehicle. The highest attainable photographic speeds and the best balances of photographic speed and image quality (hereinafter also referred to as speed-granularity relationships) have been traditionally realized with silver iodobromide emulsions. Silver bromide emulsions have been sparingly used for hand held camera photography while silver chloride containing emulsions and particularly high chloride emulsions, though clearly functional, have not found manufacturing acceptance, because of the superior performances of the other available halides. The term "high chloride" refers to grains that contain at least 50 mole percent chloride based on silver. In referring to grains of mixed halide content, the halides are named in order of increasing molar concentrations--e.g., silver iodochloride and silver iodobromide each contain a higher molar concentration of chloride or bromide, respectively, than iodide.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be achieved by employing tabular grain emulsions.
One of the very few areas in which the performance of tabular grain emulsions has not exceeded that obtainable with nontabular grain emulsions as been in the area of pressure induced alteration of photographic sensitivity. These pressure induced alterations have been observed in some instances as pressure desensitization and in other instances as pressure sensitization. Unwanted pressure effects can be induced in roll films by spooling the film strips and/or by advancing the film strip over abrading surfaces.
The shape of tabular grains renders them more vulnerable to the physical strains within the crystal structure that alter photographic response. An emulsion is generally understood to be a "tabular grain emulsion" when tabular grains account for at least 50 percent of total grain projected area. A grain is generally considered to be a tabular grain when the ratio of its equivalent circular diameter (ECD) to its thickness (t) is at least 2. The equivalent circular diameter of a grain is the diameter of a circle having an area equal to the projected area of the grain. The term "intermediate aspect ratio tabular grain emulsion" refers to an emulsion which has an average tabular grain aspect ratio in the range of from 5 to 8. The term "high aspect ratio tabular grain emulsion" refers to an emulsion which has an average tabular grain apsect ratio of greater than 8. The term "thin tabular grain" is generally understood to be a tabular grain having a thickness of less than 0.2 .mu.m. The term "ultrathin tabular grain" is generally understood to be a tabular grain having a thickness of 0.06 .mu.m or less.
The overwhelming majority of tabular grain emulsions contain tabular grains that are irregular octahedral grains. Regular octahedral grains contain eight identical crystal faces, each lying in a different {111} crystallographic plane. Tabular irregular octahedra contain two or more parallel twin planes that separate two major grain faces lying in {111} crystallographic planes. The {111} major faces of the tabular grains exhibit a threefold symmetry, appearing triangular or hexagonal. It is generally accepted that the tabular shape of the grains is the result of the twin planes producing favored edge sites for silver halide deposition, with the result that the grains grow laterally while increasing little, if any, in thickness after parallel twin plane incorporation.
While tabular grain emulsions have been advantageously employed in a wide variety of photographic and radiographic applications, the requirement of parallel twin plane formation and {111} crystal faces pose limitations both in emulsion preparation and use. These disadvantages are most in evidence in considering tabular grains containing significant chloride concentrations. It is generally recognized that silver chloride grains prefer to form regular cubic grains--that is, grains bounded by six identical {100} crystal faces. Tabular grains bounded by {111} faces in silver chloride emulsions revert to nontabular forms unless morphologically stabilized.
While tabular grain silver bromide emulsions were known to the art long before the 1980's, Wey U.S. Pat. No. 4,399,215 produced the first tabular grain silver chloride emulsion. The tabular grains were of the twinned type, exhibiting major faces of threefold symmetry lying in {111} crystallographic planes. An ammoniacal double-jet precipitation technique was employed. The thicknesses of the tabular grains were high compared to contemporaneous silver bromide and bromoiodide tabular grain emulsions because the ammonia ripening agent thickened the tabular grains. To achieve ammonia ripening it was also necessary to precipitate the emulsions at a relatively high pH, which is known to produce elevated minimum densities (fog) in high chloride emulsions. Further, to avoid degrading the tabular grain geometries sought both bromide and iodide ions were excluded from the tabular grains early in their formation.
Wey et al U.S. Pat. No. 4,414,306 developed a twinning process for preparing silver chlorobromide emulsions containing up to 40 mole percent chloride based on total silver. This process of preparation has not been successfully extended to high chloride emulsions. The highest average aspect ratio reported in the Examples was 11.
Maskasky U.S. Pat. No. 4,400,463 (hereinafter designated Maskasky I) developed a strategy for preparing a high chloride emulsion containing tabular grains with parallel twin planes and {111} major crystal faces with the significant advantage of tolerating significant internal inclusions of the other halides. The strategy was to use a particularly selected synthetic polymeric peptizer in combination with a grain growth modifier having as its function to promote the formation of {111} crystal faces Adsorbed aminoazaindenes, preferably adenine, and iodide ions were disclosed to be useful grain growth modifiers.
Maskasky U.S. Pat. No. 4,713,323 (hereinafter designated Maskasky II), significantly advanced the state of the art by preparing high chloride emulsions containing tabular grains with parallel twin planes and {111} major crystal faces using an aminoazaindene growth modifier and a gelatino-peptizer containing up to 30 micromoles per gram of methionine Since the methionine content of a gelatino-peptizer, if objectionably high, can be readily reduced by treatment with a strong oxidizing agent (or alkylating agent, King et al U.S Pat. No. 4,942,120), Maskasky II placed within reach of the art high chloride tabular grain emulsions with significant bromide and iodide ion inclusions prepared starting with conventional and universally available peptizers.
Maskasky I and II have stimulated further investigations of grain growth modifiers capable of preparing high chloride emulsions of similar tabular grain content. Tufano et al U.S. Pat. No. 4,804,621 employed di(hydroamino)azines as grain growth modifiers; Takada et al U.S. Pat. No. 4,783,398 employed heterocycles containing a divalent sulfur ring atom; Nishikawa et al U.S. Pat. No. 4,952,491 employed spectral sensitizing dyes and divalent sulfur atom containing heterocycles and acyclic compounds; and Ishiguro et al U.S. Pat. No. 4,983,508 employed organic bis-quaternary amine salts.
Bogg U.S. Pat. No. 4,063,951 reported the first tabular grain emulsions in which the tabular grains had parallel {100} major crystal faces. The tabular grains of Bogg exhibited square or rectangular major faces, thus lacking the threefold symmetry of conventional tabular grain {111} major crystal faces. In the sole example Bogg employed an ammoniacal ripening process for preparing silver bromoiodide tabular grains having aspect ratios ranging from 4:1 to 1:1. The average aspect ratio of the emulsion was reported to be 2, with the highest aspect ratio grain (grain A in FIG. 3) being only 4. Bogg states that the emulsions can contain no more than 1 percent iodide and demonstrates only a 99.5% bromide 0.5% iodide emulsion. Attempts to prepare tabular grain emulsions by the procedures of Bogg have been unsuccessful.
Mignot U.S. Pat. No. 4,386,156 represents an improvement over Bogg in that the disadvantages of ammoniacal ripening were avoided in preparing a silver bromide emulsion containing tabular grains with square and rectangular major faces. Mignot specifically requires ripening in the absence of silver halide ripening agents other than bromide ion (e.g., thiocyanate, thioether or ammonia).
Endo and Okaji, "An Empirical Rule to Modify the Habit of Silver Chloride to form Tabular Grains in an Emulsion", The JournaI of Photographic Science, Vol. 36, pp. 182-188, 1988, discloses silver chloride emulsions prepared in the presence of a thiocyanate ripening agent. Emulsion preparations by the procedures disclosed has produced emulsions containing a few tabular grains within a general grain population exhibiting mixed {111} and {100} faces.
Mumaw and Haugh, "Silver Halide Precipitation Coalescence Processes", JournaI of Imaging Science, Vol. 30, No. 5, Sep./Oct. 1986, pp. 198-299, is essentially cumulative with Endo and Okaji, with section IV-B being particularly pertinent.
Symposium: Torino 1963, Photographic Science, Edited by C. Semerano and U. Mazzucato, Focal Press, pp. 52-55, discloses the ripening of a cubic grain silver chloride emulsion for several hours at 77.degree. C. During ripening tabular grains emerged and the original cubic grains were depleted by Ostwald ripening. As demonstrated by the comparative Example below, after 3 hours of ripening tabular grains account for only a small fraction of the total grain projected area, and only a small fraction of the tabular grains were less than 0.3 .mu.m in thickness. In further investigations going beyond the actual teachings provided extended ripening eliminated many of the smaller cubic grains, but also degraded many of the tabular grains to thicker forms.
Japanese published patent application (Kokai) 02/024,643, laid open Jan. 26, 1990, was cited in a Patent Cooperation Treaty search report as being pertinent to the tabular grain structures claimed, but is in Applicant's view unrelated. The claim is directed to a negative working emulsion containing a hydrazide derivative and tabular grains with with an equivalent circular diameter of 0.6 to 0.2 .mu.m. Only conventional tabular grain preparations are disclosed and only silver bromide and bromoiodide emulsions are exemplified.
Yagi, Ito and Heki in published European patent application 466,417 Al disclose that reductions in roll film pressure desensitization can be realized when the silver halide emulsion employed contains at least 50 mole percent chloride. Although varied grain forms, including tabular grains are mentioned, it is stated that preferred grains are regular grains, which by definition excludes tabular grains. To obtain octahedral grains (that is, grains with {111} faces) it is suggested to form the grains in the presence of a spectral sensitizing dye or an inhibitor. Emulsion preparation techniques are cited only for silver chloroiodobromide emulsions and octahedral grain emulsions.